The broadcasting of digital television (DTV) signal in the United States has been done in accordance with a Digital Television Standard published in 1995 by the Advanced Television Systems Committee (ATSC) as Document A/53. An eight-level digital modulating signal controls the generation of a vestigial-sideband (VSB) signal with a suppressed very-high-frequency (VHF) or ultra-high-frequency (UHF) natural carrier, which VSB signal is transmitted together with a fixed-amplitude pilot carrier corresponding in frequency and phase with the suppressed natural carrier. The channel through which this VSB signal is transmitted with accompanying pilot carrier from the radio-frequency transmitter through the ether to the receiver is apt to include a number of component paths. Presuming there is no intervening barrier to transmission, the shortest of these paths is a direct line-of-sight path. Usually the channel will comprise a number of longer paths that result from the reflection of transmitted signal from objects outside the line-of-sight path. Multipath reception is a condition occurring when the channel includes a number of different paths, more than one of which contains sufficient energy to affect the recovery of digital modulating signal at the receiver.
The component of the broadcast DTV signal to which a DTV receiver synchronizes its operations is called the principal signal, and the principal signal is usually the strongest component of the broadcast DTV signal. The direct line-of-sight path is usually the strongest component of the broadcast DTV signal, if the direct line-of-sight path is not blocked by any intervening barrier to transmission. Therefore, the multipath signal components of the broadcast TV signal received over other paths are usually delayed with respect to the principal signal and appear as lagging multipath signals. It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes its operations to a (longer path) signal that is delayed respective to the direct signal, there will be a leading multipath component caused by the direct signal. There may also be other leading signals caused by other reflected signals of lesser delay than the signal to which the receiver synchronizes. In the DTV art the multipath components of received signal are customarily referred to as “echoes”, because of their similarity to echoes in transmission lines that are terminated other than with their characteristic impedance. The leading multipath components are referred to as “pre-echoes”, and the lagging multipath components are referred to as “post-echoes”. The echoes vary in number, amplitude and delay time from location to location and from channel to channel at a given location. Post-echoes with significant energy have been reported as being delayed from the reference signal by as many as sixty microseconds. Pre-echoes with significant energy have been reported leading the reference signal by as many as thirty microseconds. Because of these variations in echo conditions adaptive filtering is used for suppressing multipath components other than the principal signal. Such an adaptive filter is commonly referred to as an “adaptive channel-equalization filter” or an “adaptive echo-suppression filter” or, more simply as an “adaptive equalizer” or “adaptive echo suppressor”. In this specification adaptive filtering used for channel equalization and echo suppression will be referred to as “adaptive equalizer” or just “equalizer”. The adaptive filtering is customarily digital filtering. The adaptive filtering can be performed on the IF DTV signal, if the IF DTV signal is digitized. However, in most designs the adaptive filtering is performed on the digital baseband DTV signal.
The approach generally followed in DTV receiver design is down-conversion of the radio-frequency (RF) DTV signal to an intermediate-frequency (IF) DTV signal and synchronous detection of the IF DTV signal to obtain a baseband DTV signal for application to the adaptive equalizer. In some designs the synchronous detection of the IF DTV signal is done in the analog regime, with the resulting analog baseband DTV signal being digitized for application to the adaptive equalizer. In other designs the IF DTV signal is digitized and synchronous detection of the digitized IF DTV signal is done in the digital regime, to generate the digital baseband DTV signal applied to the adaptive equalizer.
In some prior-art designs the adaptive equalizer is operative on a real-only baseband DTV signal. This signal is supplied from an in-phase synchronous detector that synchronously detects the IF DTV signal in accordance with a carrier that is synchronized with the suppressed carrier of the IF DTV signal and with the corresponding pilot carrier as converted to intermediate frequency. In many of these designs the real-only baseband DTV signal is sampled at twice Nyquist rate and the adaptive equalizer is of fractional type. In others of these designs, the adaptive equalizer is of synchronous type, with the real-only baseband DTV signal being subjected to a procedure known as phase-tracking before being sampled at Nyquist rate to supply input signal to the adaptive equalizer.
Other prior-art designs employ an adaptive equalizer that is complex in nature, being operative not only on a real component of baseband DTV signal supplied from an in-phase synchronous detector, but also on an imaginary component of baseband DTV signal supplied from an in-phase synchronous detector. J. D. McDonald and A. L. R. Limberg describe an alternative type of adaptive equalizer that is complex in nature in U.S. Pat. No. 6,975,689 issued 13 Dec. 2005 from application Ser. No. 09/823,500 filed 30 Mar. 2001. The patent and application are both titled “DIGITAL MODULATION SIGNAL RECEIVER WITH ADAPTIVE CHANNEL EQUALIZATION EMPLOYING DISCRETE FOURIER TRANSFORMS”. In this alternative type of adaptive equalizer synchronous detection is performed nominally at −45° phasing and at +45° phasing respective to the suppressed carrier of the IF DTV signal. The adaptive equalizer has respective portions for equalizing the response of the synchronous detector detecting at a nominally −45° carrier phase and for equalizing the response of the synchronous detector detecting at a nominally +45° carrier phase. The responses of these two portions of the adaptive equalizer are additively combined to recover a real baseband DTV signal and are differentially combined to recover an imaginary baseband DTV signal. The imaginary baseband DTV signal is lowpass filtered to develop an automatic phase control (APC) signal for the carrier generator generating −45°-phase and +45°-phase carrier signals used by the synchronous detectors. The result of performing synchronous detection at −45° phasing is the negative of the result of performing synchronous detection at +135° phasing as described herein.
U.S. Pat. No. 5,065,242 titled “DEGHOSTING APPARATUS USING PSEUDORANDOM SEQUENCES” issued 23 Aug. 1994 to Charles Dietrich and Arthur Greenberg. Pat. No. 5,065,242 describes the computation of the weighting coefficients of adaptive filters used for deghosting analog DTV signals, which computation is based on measurements of the channel impulse response (CIR) made during the reception of triple-PN127 sequences in a vertical-retrace-interval scan line. U.S. Pat. No. 6,975,689 describes adaptive equalizers in which weighting coefficients of the equalization filters are calculated from discrete Fourier transforms (DFTs) of data. Earlier adaptive equalizers for DTV receivers used auto-regression techniques based on the detection of reception error for adjusting the weighting coefficients of the adaptive equalizer by reducing the gradient of departures of equalizer response from ideal transmission symbols. In a procedure that is novel, the DTV receivers described in this specification use the detection of reception error for updating an initial measurement of CIR to track current reception conditions. Periodically, the weighting coefficients of the equalization filters are re-calculated based on a strobe of the continually updated CIR.
The synchronization of DTV receiver operations to the principal signal has been a source of long-standing problems for receiver designers. These problems have to do with developing an echo-free baseband DTV signal that is optimally sampled at baud rate, so each successive symbol of the recovered digital modulating signal experiences as little intersymbol interference (ISI) from preceding or succeeding samples as possible when subjected to data slicing procedures. Constraining the baseband DTV signal to Nyquist bandwidth makes the elimination of ISI possible, providing correct symbol synchronization can be achieved. That is, data slicing requires that sampling of the echo-free baseband DTV signal at the baud rate of the symbols be done in the exact phasing that minimizes ISI.
U.S. Pat. No. 5,479,449 titled “DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER” issued 26 Dec. 1995 to C. B. Patel and A. L. R. Limberg. U.S. Pat. No. 5,479,449 describes a symbol synchronization procedure that adjusts digital oversampling of a baseband DTV signal to optimize phasing for decimation to Nyquist rate. The oversampling samples preceding peak oversampling samples are accumulated, the oversampling samples succeeding peak oversampling samples are accumulated, and the accumulation results are compared to determine whether the oversampling clock should be advanced or retarded in phase so peak samples recovered at Nyquist rate through decimation are optimally phased. S. U. H. Qureshi in his paper “Timing Recovery for Equalized Partial-Response Systems”, IEEE TRANSACTIONS ON COMMUNICATIONS, December 1976, pp. 1326–1330 earlier described a similar method used for symbol synchronization of QAM signals. The adjustment of sample phasing moves the data-slicer window together with phasing of the Nyquist sample in these previously employed methods.
The DTV receivers described in this specification use a different method of symbol synchronization, in which the data-slicer window recurs at a predetermined phasing of the Nyquist sampling clock. A twice-Nyquist-rate sampling clock is determined from the symbol clock rate, as recovered by bright-line spectral recovery techniques. The digital baseband DTV signal is sampled in accordance with the twice-Nyquist-rate sampling clock. The oversampled digital baseband DTV signal is decimated by alternate sample selection techniques to generate two digital baseband DTV signals, each sampled at Nyquist rate by a sampling clock that is in staggered phasing with respect to the sampling clock of the other. These two digital baseband DTV signals, each sampled at Nyquist rate, are re-timed in accordance with the same Nyquist-rate sampling clock as will be used for data slicing and are then synchronously equalized independently of each other. The equalizer results are additively combined in preparation for data slicing, which additive combining is done in proportions subject to adjustment. The adjustment is made so that the additively combined equalizer results resample the synchronously equalized digital baseband DTV signal to be in temporal alignment with the data-slicer window that recurs at predetermined phasing of the Nyquist sampling clock. In effect, in DTV receivers described in this specification, the phase of the digital baseband DTV signal is adjusted vis-à-vis recurring data-slicer windows of predetermined phasing. This is done rather than following the practice in prior-art DTV receivers in which practice the data-slicer window is adjusted in phase to attempt to quantize the digital baseband DTV signal optimally.